The invention relates to high temperature fuel cells and, more particularly, to fuel cells having internal catalytic auto thermal reforming of fuel.
An electrochemical fuel cell converts the chemical bond energy potential of fuel to electrical energy in the form of direct current (DC) electricity. Fuel cells are presently being considered as replacement for battery storage systems and conventional electric generating equipment.
A fuel cell stack is comprised of a plurality of individual fuel cells stacked together and arranged in an electrical series relationship to produce higher useable DC voltage. A DC/AC inverter may be utilized to convert the DC electrical current to AC electrical current for use in common electrical equipment.
A fuel cell stack formed of Molten Carbonate Fuel Cells (MCFC""s) typically operates at about 650xc2x0 C. This high temperature provides the opportunity for the fuel cell stack to operate at high efficiency using a variety of hydrocarbon-based fuel feed stocks.
All fuel cells utilize diatomic hydrogen in an electrochemical fuel cell reaction. The hydrogen may be derived from a variety of hydrocarbon-based fuel feed stocks, such as methane and methanol. The derivation of hydrogen from hydrocarbon-based fuel feed stock is achieved by the process of reforming. Reforming of hydrocarbon fuels may be achieved by several means. Catalytic Steam Reforming (CSR), Catalytic Partial Oxidation (CPOX) reforming, and Catalytic Auto Thermal Reforming (CATR) are widely known in the art as methods used to reform hydrocarbon fuel. CATR is known as the coupling of CSR with CPOX. CATR has been further defined as a CSR reaction and a CPOX reaction that occur over microscopic distances at a common catalytic site, thus avoiding complex heat exchange. CATR has further been defined in the art as occurring when there is no wall between a combined CSR reaction and a catalyzed CPOX reaction.
It is desirable to strive for 100% conversion of the fuel feed stock in the reforming process. Since un-converted fuel feed stock will not react with the anode electrode of an MCFC stack, full conversion avoids the passage of non-useable fuel feed stock through the fuel cell. Passage of unconverted fuel feed stock through the fuel cell, therefore, has the effect of diminishing the efficiency of the fuel cell through under-utilization of the fuel feed stock. Carbon monoxide produced in the reforming process is useable as a fuel in an MCFC stack.
It is well known in the art that the method of internally reforming methane fuel feed stock within an MCFC, as taught by U.S. Pat. Nos. 3,488,226 and 4,182,795, to Baker et al., couples the exothermic fuel cell reaction with the endothermic stream reforming reaction. This method has become known in the art as Direct Internal Reforming (DIR).
However, DIR reforming of fuel in an MCFC stack presents significant difficulties. For example, a typical method of DIR utilizes a nickel catalyst on a magnesium oxide substrate in pelletized form. This form of catalyst is loaded into the anode flow chamber of the active area of the fuel cell. The nickel catalyst on the surface of the pellets possesses an extremely high surface area. The electrolyte of a carbonate fuel cell is highly mobile through both surface creepage as well as evaporation into the gas stream. This mobile electrolyte contaminates the nickel on the surface of the pellet, and, therefore, the high surface area nickel catalyst rapidly becomes non-functional. This results in excessive quantities of non-reformed fuel feed stock slipping past the catalyst and exiting the fuel cell without having been utilized in the fuel cell reaction, thereby diminishing the fuel efficiency of the fuel cell. This decay of the reforming catalyst typically will occur sooner than that of other components within the fuel cell, and results in premature failure of the fuel cell system.
Another technique applied to internal reforming of fuel gas within a carbonate fuel cell utilizes a separate chamber for the catalyst, as taught by the Baker et al. patents, as well as U.S. Pat. No. 5,175,062 to Farooque et al. This method, known as Indirect Internal Reforming (IIR), is effective in avoidance of electrolyte contamination but fails to achieve the beneficial effects of the close coupling of endothermic/exothermic reactions that occurs in direct internal reforming. The fuel gas cannot achieve 100% reforming conversion within the IIR chamber of an MCFC operating at 650xc2x0 C. Typically, the partially reformed fuel is polished using a DIR catalyst that remains subject to electrolyte contamination and premature failure.
Reforming of fuel feed stock external to the fuel cell stack may take many forms. U.S. Pat. No. 4,902,586 to Werthiem teaches an Auto Thermal Reformer (ATR) for an MCFC, external to the confines of the fuel cell, that utilizes the cathode exhaust as the source of oxidant for the combustion reaction in the ATR. However, it is known that high temperature fuel cells such as MCFC""s benefit from the close coupling of the exothermic and endothermic reactions of the fuel cell and reformer. For example, U.S. Pat. No. 5,366,819 to Hartvigsen et al. teaches an ATR thermally integrated within the confines of the insulated walls of a high temperature Solid Oxide Fuel Cell (SOFC). U.S. Pat. No. 5,079,105 to Bossel teaches the application of a reforming device centrally located within an arrangement of four fuel cell stacks. Heat is transferred to the reforming device by the recirculation of gaseous media and the radiated Joule heat accumulating in the fuel cell by ohmic losses.
It is an object of the present invention to provide a fuel cell having an internal thermally integrated autothermal reformer that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of preferred embodiments.
Accordingly, it is seen as desirable to provide an improved MCFC system that utilizes a Catalytic Auto Thermal Reformer (CATR) that is internally positioned within the fuel cell stack and is thermally integrated to improve the efficiency of the CATR process by reducing the quantity of oxygen required to elevate and maintain the operational temperature of the CATR.
In accordance with a first aspect, an apparatus for auto thermal reforming hydrocarbon fuel in a fuel cell stack includes a plurality of fuel cells stacked together. Each fuel cell has an inlet manifold, and the inlet manifolds of the fuel cells are aligned with one another to form a manifold chamber. A porous wand is positioned within the manifold chamber. A mixing device is positioned within the wand and is configured to carry a fuel gas and an oxidant through the wand.
In accordance with another aspect, an apparatus for reforming hydrocarbon fuel in a fuel cell stack includes a plurality of fuel cells stacked together. Each fuel cell includes a bipolar separator plate having an inlet manifold and an outlet manifold. The inlet manifolds of the fuel cells in the stack are aligned with one another to form a manifold chamber. A tubular porous wand is positioned in the manifold chamber. A mixing device is positioned within the wand and has a first passageway configured to carry a fuel gas and a second passageway configured to carry an oxidant. A catalyst is deposited on the porous wand to promote reforming of a fuel gas.
In accordance with yet another aspect, an apparatus for reforming hydrocarbon fuel in a fuel cell stack includes a plurality of fuel cells stacked together. Each fuel cell includes an anode electrode, a cathode electrode, an electrolyte matrix, and a bipolar separator plate having an inlet manifold and an outlet manifold. The inlet manifolds of the fuel cells in the stack are aligned with one another to form a manifold chamber. A tubular porous wand is positioned in the manifold chamber. A mixing device includes a tubular member having an internal wall defining a first passage configured to carry a fuel gas and a second passage configured to carry an oxidant. A catalyst is deposited on the porous wand to promote catalytic auto thermal reforming of a fuel gas
From the foregoing disclosure, it will be readily apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this area of technology, that the present invention provides a significant advance. Preferred embodiments of the present invention can provide improved reforming of fuel feed stock and improved fuel cell efficiencies. These and additional features and advantages of the invention disclosed here will be further understood from the following detailed disclosure of preferred embodiments.